This invention relates to circulating air dryers, such as might be employed in the drying of lumber. While the drying of lumber is specifically discussed herein, it should be understood that the dryer of the invention is not limited to the drying of lumber, as other products may be equally well dried using the principles contemplated.
In a conventional dryer, a system of fans inside a kiln or drying chamber maintains a more or less uniform flow of air through the lumber. The moving air supplies heat to the wood and removes the wood's moisture which is vented to the outside of the kiln. The wet bulb and dry bulb temperatures of the air passing through the lumber are controlled, and various drying schedules are used depending upon the moisture content of the wood, the wood species, and the end use of the lumber. In a typical kiln, moist air is exhausted through roof vents and as humid air is exhausted, drier colder outside air is drawn into the kiln which is heated within the kiln to operating temperature.
A conventional dryer wastes energy in several ways. For instance, humid air often is exhausted after it passes through heating coils within the dryer. This adds energy to the exhaust air, and this energy is wasted. In some dryers, after the humid air passes through heating coils, it is diluted and chilled with cold outside air and then recirculated. This system tends to waste energy, because originally humid air is heated and diluted before being exhausted. Complicating the problem of efficient use of energy is that in many dryers the direction of air flow through the dryer is reversed periodically. To have an efficient dryer with uniform operation, the way that make-up or dilution air is handled, humid air is exhausted, and heat is supplied to circulated air, should be in an essentially uniform manner, irrespective of the direction of the circulated air flow.
In Cook, Re Issue U.S. Pat. No. 28,226, a circulating air dryer is disclosed which has a construction obviating many of the disadvantages of conventional dryers. However, problems still exist with circulating air dryers, especially as used in multiple load applications, such as those known as a multi-track lumber kilns where circulated air passes through first one load and then another, and the humidity of the air rises on the air passing through the first load.
A general object of this invention, therefore, is to provide improvements in a circulating air dryer which contribute to a more efficient use of energy, and which also contribute to a more uniform type of drying than is obtainable with prior known dryers.
More specifically, an object is to provide an improved circulating air dryer where air flow through the dryer is periodically reversed, through reversal of an air mover or other instrumentality, and where a more efficient use of energy and more uniform dryer characteristics are obtainable, irrespective of the direction of air movement.
A further object is the provision of a circulating air dryer, specifically applicable to the drying of multiple loads, where air is circulated through the loads in either of opposite directions along a path, and where there is exhausting of humid air and introduction of dilution air in a zone disposed between the loads.
A related object is to provide such a construction which further includes means in the zone between loads for supplying heat to circulated air, and wherein the means for supplying dilution air and the means for extracting humid air are constructed in such a manner as to provide a uniform type of operation irrespective of the direction of air movement.